The present invention relates to an apparatus for measuring the electromagnetic radiation absorbed by gaseous and particulate matter on exposure to light and, more particularly, to a spectrophone assembly which is capable of simultaneously and accurately measuring individual atmospheric particulate and gaseous radiation absorptions in situ.
The electromagnetic radiation absorption properties of gases and particulate substance have proven increasingly important to such diverse interests as high energy laser propagation, electro-optical sensors, communications by optical systems, the control of atmospheric particulate matter and other radiation absorbing atmospheric pollutants. Since more atmospheric gaseous absorbers are unstable (e.g. ozone and the various nitrogen oxides) and some atmospheric aerosol particulates are known to contain significant amounts of volatile components (and therefore to be rather tenuous in nature) it is desirable to measure their absorption in situ. Measurement of aerosol particulate matter absorption in situ is difficult and has to date only been done definitively by rather indirect techniques. The spectrophone of the subject invention makes the necessary measurements more directly, bypassing altogether the previously required measurements of particle size, shape, and complex refractive index. The principal advantage in utilizing a spectrophone to measure these properties is the potential for higher sensitivity than may be obtained with more traditional comparative path transmission measurements.
The prior art describes a resonant spectrophone which measures radiation absorption by exposing the medium to be tested to a laser beam or other high intensity light source and measuring the energy absorbed by the gaseous or particulate medium in an acoustically tuned absorption chamber using a microphone as an audio sensor. However, the optical energy absorbed by the end windows of the spectrophone chamber produces an undesirable acoustical signal for transmission into the medium being measured. In the past, this spurious signal has rendered highly accurate measurements by spectrophone unattainable and, as a result, it has not been possible to use the spectrophone for a comprehensive analysis of airborne pollution comprising both gaseous and particulate constituents of different absorption properties.